LCD display screen has been widely used in various display fields, such as home, public places, offices and personal electronics-related products. As people's increasing demand for display, touch operation has been used more and more widely. Touch screen is usually implemented as resistive, capacitive, optical, acoustic wave, etc., in which the capacitor is embedded in the touch screen, i.e., the embedded capacitive touch manner can make the touch screen to be thinner, lighter than the touch screen of other touch manner, and save more cost, and therefore the embedded capacitive touch screen draws more and more attention.
FIG. 1 is an embedded capacitive touch sensing circuit in the prior art, as shown in FIG. 1, the circuit comprises: a first transistor M1, a second transistor M2, an amplification transistor M3, a fixed capacitor C1 and an variable capacitor C1c, wherein the gate of the first transistor M1 is connected to the first control line, the source of the first transistor M1 is connected to the voltage line, the drain of the first transistor M1 is connected to the node c, the first transistor M1 is used to implement the initialization process. One terminal of the fixed capacitor C1 is connected to the first control line, and the other terminal of the fixed capacitance C1 is connected to the node c, so as to hold the gate-drain voltage of the first transistor M1. One terminal of the variable capacitor C1c is connected to the node c, the other terminal of the variable capacitor C1c is connected to the bias line. The gate of the amplification transistor M3 is connected to the node c, the source of the amplification transistor M3 is connected to the voltage line, the drain of the amplification transistor M3 is connected to the source of the second transistor M2; the gate of the second transistor M2 is connected to the second control line, the drain of the second transistor M2 is connected to the read-out line. The operation of the circuit is as follows.
When the first control line is applied with high level, the first transistor M1 is turned on, the fixed capacitor C1 and the variable capacitor C1c are charged, the node c is charged to V1; when the first control line is applied with low level, due to the capacitive coupling effects, the voltage on node c becomes: Vc=V1−C1*ΔVp/(C1+C1c), wherein ΔVp is the difference value between the pulse high voltage and low voltage on the first control line. When a touch occurs, the variable capacitance C1c changes, the node voltage Vc is changed, resulting in a change in the gate voltage of the amplification transistor M3, thereby the source current flowing through the amplification transistor M3 is changed, i.e. the source current from the amplification transistor M3 to the second transistor M2 is changed. When the second control line is applied with high level, the second transistor M2 is turned on, the current from the drain of the second transistor M2 to the read-out line is changed, and the changed current is detected by the detection unit connected with the read-out line, such that a detection of the touch location and subsequent operations can be performed.
Those problems exist in the prior art: compared to the Vc value when no touch occurs, the change in Vc value generated by change of the variable capacitor C1c when a touch occurs is relative small, change in the current flowing through the amplification transistor M3 and the second transistor M2 to the read-out line is small, such that the accuracy of the touch sensing circuit in determining the occurrence of touch is not satisfactory.